Most people who will buy an older, imported English gun grasp the fact that the guns have specific ammunition requirements. This is detailed in the markings on the flats. Even if proof specs change, that does not negate that the gun was proofed to a certain level, and ammunition that meets the requirements of that level of proof, should be used in that gun. Hint: Most American loaded ammunition is NOT intended to meet English shotgun requirements. Measuring the bores, chokes and chambers will show if someone, say, an unscrupulous US dealer, has had them tampered with, by excessive honing or boring, or otherwise changing the dimensions since the gun was proofed. Reproof, does not mean it is ready to run here in the states on promo loads from Wal-Mart.

Why do you have such a tough time grasping this, ed?

This isn’t that tough.
Best,
Ted